61,799 research outputs found
Moduli Vacuum Bubbles Produced by Evaporating Black Holes
We consider a model with a toroidally compactified extra dimension giving
rise to a temperature-dependent 4d effective potential with one-loop
contributions due to the Casimir effect, along with a 5d cosmological constant.
The forms of the effective potential at low and high temperatures indicates a
possibility for the formation of a domain wall bubble, formed by the modulus
scalar field, surrounding an evaporating black hole. This is viewed as an
example of a recently proposed black hole vacuum bubble arising from
matter-sourced moduli fields in the vicinity of an evaporating black hole [D.
Green, E. Silverstein, and D. Starr, Phys. Rev. D74, 024004 (2006),
arXiv:hep-th/0605047]. The black hole bubble can be highly opaque to lower
energy particles and photons, and thereby entrap them within. For high
temperature black holes, there may also be a symmetry-breaking black hole
bubble of false vacuum of the type previously conjectured by Moss [I.G. Moss,
Phys. Rev. D32,1333 (1985)], tending to reflect low energy particles from its
wall. A double bubble composed of these two different types of bubble may form
around the black hole, altering the hole's emission spectrum that reaches
outside observers. Smaller mass black holes that have already evaporated away
could have left vacuum bubbles behind that contribute to the dark matter.Comment: 20 pages; to appear in Phys.Rev.
Visual attitude orientation and alignment system
Active vehicle optical alignment aid and a passive vehicle three-dimensional alignment target ensure proper orientation and alignment plus control of the closure range and rate between two bodies, one in controlled motion and one at rest
Background independent exact renormalization group for conformally reduced gravity
Within the conformally reduced gravity model, where the metric is
parametrised by a function of the conformal factor , we keep
dependence on both the background and fluctuation fields, to local potential
approximation and respectively, making no other
approximation. Explicit appearances of the background metric are then dictated
by realising a remnant diffeomorphism invariance. The standard non-perturbative
Renormalization Group (RG) scale is inherently background dependent, which
we show in general forbids the existence of RG fixed points with respect to
. By utilising transformations that follow from combining the flow equations
with the modified split Ward identity, we uncover a unique background
independent notion of RG scale, . The corresponding RG flow equations
are then not only explicitly background independent along the entire RG flow
but also explicitly independent of the form of . In general is
forced to be scale dependent and needs to be renormalised, but if this is
avoided then -fixed points are allowed and furthermore they coincide with
-fixed points.Comment: 53 pages, broken reference correcte
Some effects of small-scale metallicity variations in cooling flows
In an attempt to reconcile recent spectral data with predictions of the
standard cooling flow model, it has been suggested that the metals in the
intracluster medium (ICM) might be distributed inhomogeneously on small scales.
We investigate the possible consequences of such a situation within the
framework of the cooling flow scenario. Using the standard isobaric cooling
flow model, we study the ability of such metallicity variations to
preferentially suppress low-temperature line emission in cooling flow spectra.
We then use simple numerical simulations to investigate the temporal and
spatial evolution of the ICM when the metals are distributed in such a fashion.
Simulated observations are used to study the constraints real data can place on
conditions in the ICM. The difficulty of ruling out abundance variations on
small spatial scales with current observational limits is emphasized. We find
that a bimodal distribution of metals may give rise to interesting effects in
the observed abundance profile, in that apparent abundance gradients with
central abundance drops and off-centre peaks, similar to those seen recently in
some clusters, are produced. Different elements behave in different fashion as
governed by the temperature dependence of their equivalent widths. Our overall
conclusion is that, whilst this process alone seems unlikely to be able to
account for the sharp reduction in low temperature emission lines seen in
current spectral data, a contribution at some level is possible and difficult
to rule out. The possibility of small-scale metallicity variations should be
considered when analysing high resolution cluster X-ray spectra.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Redundant operators in the exact renormalisation group and in the f(R) approximation to asymptotic safety
In this paper we review the definition and properties of redundant operators in the exact renormalisation group. We explain why it is important to require them to be eigenoperators and why generically they appear only as a consequence of symmetries of the particular choice of renormalisation group equations. This clarifies when Newton’s constant and or the cosmological constant can be considered inessential. We then apply these ideas to the Local Potential Approximation and approximations of a similar spirit such as the f (R) approximation in the asymptotic safety programme in quantum gravity. We show that these approximations can break down if the fixed point does not support a ‘vacuum’ solution in the appropriate domain: all eigenoperators become redundant and the physical space of perturbations collapses to a point. We show that this is the case for the recently discovered lines of fixed points in the f (R) flow equations
Asymptotic safety in the f(R) approximation
In the asymptotic safety programme for quantum gravity, it is important to go
beyond polynomial truncations. Three such approximations have been derived
where the restriction is only to a general function f(R) of the curvature R>0.
We confront these with the requirement that a fixed point solution be smooth
and exist for all non-negative R. Singularities induced by cutoff choices force
the earlier versions to have no such solutions. However, we show that the most
recent version has a number of lines of fixed points, each supporting a
continuous spectrum of eigen-perturbations. We uncover and analyse the first
five such lines. Sensible fixed point behaviour may be achieved if one
consistently incorporates geometry/topology change. As an exploratory example,
we analyse the equations analytically continued to R<0, however we now find
only partial solutions.We show how these results are always consistent with,
and to some extent can be predicted from, a straightforward analysis of the
constraints inherent in the equations.Comment: Latex, 66 pages, published version, typos correcte
Leak test system
System for quantitative determination of leak rates in large pressurized compartments is described. Method uses pressure reference cylinder placed in thermal contact with internal environment of compartment. Construction of equipment and details of operational procedure are reported. Illustration of equipment is included
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